Such a process is described in EP-A-257719. This publication describes a process for cooling a hot gas, wherein also super heated steam is formed. With super heated steam is meant steam having a higher temperature than its saturation temperature. EP-A-257719 describes a vessel consisting of a primary evaporation tube bundle for passage of the hot gas. This tube bundle is submerged in a space of water. In use steam will form when hot gas passes the tube bundle. This steam is fed to a super heater module, consisting of a shell-tube heat exchanger, which is submerged in the same space of water. In this module partially cooled gas from the primary evaporator tube bundle is fed to the shell side of the superheater module and the steam is fed to the tube side of the superheater module. The two flows are contacted in the superheater in a co-current mode of operation.
Applicants found that when the process according to EP-A-257719 is used to cool gas comprising contaminants such as carbon, ash and/or sulphur, which is for example the case for synthesis gas produced by gasification of a gaseous or liquid hydrocarbonaceous feedstock, leakage can occur. It is believed that fouling of the apparatus at the gas side causes leakage. Although the apparatus was cleaned regularly the leakage problems persisted. Fouling, especially when the synthesis gas is produced by gasification of a liquid hydrocarbon, in particular heavy oil residues, will also result in that the heat exchange capacity of the apparatus will gradually decrease with run time. As a result, the temperature of the process gas leaving the heat exchanger will increase gradually with runtime. If the temperature of the process gas leaving the heat exchanger apparatus exceeds a certain temperature, typically 400-450° C., the temperature of the tubes that transmit the process gas downstream of the heat exchanger will be so high that they may be damaged. Therefore, the apparatus has to be shut down in order to clean the tubes. The runtime of an apparatus after which the tubes have to be cleaned is referred to as ‘cycle time’.